And that is a wonderful question. I presently have every profession in the game maxed with the exception of Leather working/skinning. That will be fixed when my Monk finally gets to max level however. (As a side not, leveling leather working so far has been the worst profession grind in my opinion.) With Warlords of Draenor, professions will get another 75 point kick, if they stay consistent. That means if you start a new character that is 675 points of professions to work through to max any given profession. That is a little daunting.
So first I would suggest adding in profession quests. Cooking has one of these, so does fishing. Imagine however if you had a daily/weekly quest you could take based around your profession to gain +5 or +10 points for completing it. Only one or two points if it's a daily. I know we have a monthly +5 we can get from the Darkmoon fair, but with that being just once a month it doesn't feel like that big of a kick up.
Second I would add more meaningful/fun items. I have two engineers. I wouldn't mind having more. Why? Engineers are like freaking Batman! You get belt rockets that help you to run on water, a glider attached to your cloak and other fun toys like rocket boots etc.. that you can play with that gives your character a nice flavor. It however has the save disadvantage a lot of the other professions have.
There are gaps where it feels useless....
With each profession you get to make raid entry level gear. Then you have to wait for the next raid to come out, for new gear that is insanely expensive to make. This gear's only advantage is that it brings you up to the gear level of whatever the current raid level of gear is. Normally by the time you can make it, you've been running the raid for a few weeks and in all likely hood a hard core raider is going to have something comparable all ready, and any one who isn't pushing hard against the raid is unlikely to care to drop tons of gold on a one gear piece boost.
So first I think we should be able to get some simpler to make pieces with each new raid tier. I know that Blizzard wants to gate things, but you can make equipment difficult to make without making people need to buy whatever item is dropping from the new raid to complete the items. Daily cool downs, rarely found/picked items all this can be used to gate new equipment.
I think there should also be more gear made purely for transmog. Since this has become a well know used and loved feature, there should be gear that can only be used/mogged by members of said profession, and other items with some flare that can be used by everyone who would normally be able to use such armor type (Mail, Plate etc...)
Then we come to gathering professions.
I think player should, for a fee or quest, be able to get assistants who farm stuff for them. I like the farms in Panderia at Sunsong Ranch, but the problem is that eventually it just feels grindy. Give me a companion in my garrison who I can send to gather mats, knowing that sometimes he will come back with a very low yield, and occasionally he will come back with some cool stuff. I know farming mats is part of the game, and you could still do that while using this feature to speed up your gathering ability. You could also gain rep with different farmers you send on quests to get better yields and possible pets/mounts.
Anyway, I would love more stuff to keep professions interesting and profitable. Obviously some gates have to exist to keep the gear from all falling to dirt cheap prices right away, or making the professions meaningful. I do believe however, that some of these gates could creak open a little more, or even if your looking at past Xpack gear that a way to speed through non-current/useful patterns/items could be nice.
The real question is, how would you fix professions?